(NB: If this question starts out looking very familiar to you, this is because it is in fact my second question about the same snippet of text. The first one is this one: "andern" vs "anderen" .)
The citation below is the Duden's definition of vorausfahren (my emphasis):
schon vorher, früher als ein anderer oder vor [einem] andern her irgendwohin fahren
Q: What exactly does the highlighted her add to the meaning of the sentence above? In other words, what effect would it have to omit it?
(PS: Any reference work that you may be able cite in support of your answer would be much appreciated.)